1. Field of the Invention
The present invention relates to a helicopter blade aerofoil and a helicopter blade.
2. Description of the Related Art
In Japanese Patent Application No. 9-30730 (Japanese Unexamined Patent Publication JP-A 10-226397 (1998)) corresponding to U.S. Pat. No. 5,961,290, the Applicant of the present invention proposed an aerofoil AK100D as the prior application having large maximum lift coefficient Clmax and lift-drag ratio and reduced noise level.
Another prior art of the helicopter blade aerofoils are known Japanese Unexamined Patent Publications JP-A 50-102099 (1975), JP-A 59-134096 (1984), JP-A 63-64894 (1988), Japanese Examined Patent Publication JP-B2 62-34600 (1987), Japanese Patent Application No. 8-52121 (Japanese Unexamined Patent Publication JP-A 9-240593 (1997)) corresponding to U.S. Pat. No. 5,957,662, etc.
The aerofoil AK100D (the prior application), having excellent lift characteristics, drag characteristics, and lift-drag characteristics, especially superior lift characteristics at low velocities, in which a reduced noise level can be achieved. However it has a tendency that the maximum lift coefficient in a middle velocity range of Mach numbers M of 0.5 to 0.6 is small.
When a helicopter with a rotor having a radius R which is rotating at a rotational angular velocity .OMEGA. flies at a ground velocity V, the airspeeds of an advancing blade where the rotational speed of the rotor is added to the ground velocity V, and a retreating blade where the ground velocity V is subtracted from the rotational speed of the rotor are significantly different from each other. The operating envelope of helicopter blades thus exist in a wide range from lower velocity to higher velocity. In order to improve the performance of the rotor as a whole, therefore, it is necessary to improve the lift characteristics in wider velocity range.